W-CDMA (Wideband Code Division Multiple Access) is a radio interface for IMT-2000 (International Mobile Communication), which was standardized for use as the 3rd generation wireless mobile telecommunication system. It provides a variety of services such as voice services and multimedia mobile communication services in a flexible and efficient way. The standardization bodies in Japan, Europe, USA, and other countries have jointly organized a project called the 3rd Generation Partnership Project (3GPP) to produce common radio interface specifications for W-CDMA.
The standardized European version of IMT-2000 is commonly called UMTS (Universal Mobile Telecommunication System). The first release of the specification of UMTS has been published in 1999 (Release 99). In the mean time several improvements to the standard have been standardized by the 3GPP in Release 4 and Release 5 and discussion on further improvements is ongoing under the scope of Release 6.
The dedicated channel (DCH) for downlink and uplink and the downlink shared channel (DSCH) have been defined in Release 99 and Release 4. In the following years, the developers recognized that for providing multimedia services—or data services in general—high speed asymmetric access had to be implemented. In Release 5 the high-speed downlink packet access (HSDPA) was introduced. The new high-speed downlink shared channel (HS-DSCH) provides downlink high-speed access to the user from the UMTS Radio Access Network (RAN) to the communication terminals, called user equipments in the UMTS specifications.
Hybrid ARQ Schemes
The most common technique for error detection of non-real time services is based on Automatic Repeat reQuest (ARQ) schemes, which are combined with Forward Error Correction (FEC), called Hybrid ARQ. If Cyclic Redundancy Check (CRC) detects an error, the receiver requests the transmitter to send additional bits or a new data packet. From different existing schemes the stop-and-wait (SAW) and selective-repeat (SR) continuous ARQ are most often used in mobile communication.
A data unit will be encoded before transmission. Depending on the bits that are retransmitted three different types of ARQ may be defined.
In HARQ Type I the erroneous data packets received, also called PDUs (Packet Data Unit) are discarded and new copy of that PDU is retransmitted and decoded separately. There is no combining of earlier and later versions of that PDU. Using HARQ Type II the erroneous PDU that needs to be retransmitted is not discarded, but is combined with some incremental redundancy bits provided by the transmitter for subsequent decoding. Retransmitted PDU sometimes have higher coding rates and are combined at the receiver with the stored values. That means that only little redundancy is added in each retransmission.
Finally, HARQ Type III is almost the same packet retransmission scheme as Type II and only differs in that every retransmitted PDU is self-decodable. This implies that the PDU is decodable without the combination with previous PDUs. In case some PDUs are heavily damaged such that almost no information is reusable self decodable packets can be advantageously used.
When employing chase-combining the retransmission packets carry identical symbols. In this case the multiple received packets are combined either by a symbol-by-symbol or by a bit-by-bit basis (see D. Chase: “Code combining: A maximum-likelihood decoding approach for combining an arbitrary number of noisy packets”, IEEE Transactions on Communications, Col. COM-33, pages 385 to 393, May 1985). These combined values are stored in the soft buffers of respective HARQ processes.
Packer Scheduling
Packet scheduling may be a radio resource management algorithm used for allocating transmission opportunities and transmission formats to the users admitted to a shared medium. Scheduling may be used in packet based mobile radio networks in combination with adaptive modulation and coding to maximize throughput/capacity by e.g. allocating transmission opportunities to the users in favorable channel conditions. The packet data service in UMTS may be applicable for the interactive and background traffic classes, though it may also be used for streaming services. Traffic belonging to the interactive and background classes is treated as non real time (NRT) traffic and is controlled by the packet scheduler. The packet scheduling methodologies can be characterized by:                Scheduling period/frequency: The period over which users are scheduled ahead in time.        Serve order: The order in which users are served, e.g. random order (round robin) or according to channel quality (C/I or throughput based).        Allocation method: The criterion for allocating resources, e.g. same data amount or same power/code/time resources for all queued users per allocation interval.        
The packet scheduler for uplink is distributed between Radio Network Controller (RNC) and user equipment in 3GPP UMTS R99/R4/R5. On the uplink, the air interface resource to be shared by different users is the total received power at a Node B, and consequently the task of the scheduler is to allocate the power among the user equipment(s). In current UMTS R99/R4/R5 specifications the RNC controls the maximum rate/power a user equipment is allowed to transmit during uplink transmission by allocating a set of different transport formats (modulation scheme, code rate, etc.) to each user equipment.
The establishment and reconfiguration of such a TFCS (transport format combination set) may be accomplished using Radio Resource Control (RRC) messaging between RNC and user equipment. The user equipment is allowed to autonomously choose among the allocated transport format combinations based on its own status e.g. available power and buffer status. In current UMTS R99/R4/R5 specifications there is no control on time imposed on the uplink user equipment transmissions. The scheduler may e.g. operate on transmission time interval basis.
UMTS Architecture
The high level R99/4/5 architecture of Universal Mobile Telecommunication System (UMTS) is shown in FIG. 1 (see 3GPP TR 25.401: “UTRAN Overall Description”, available from http://www.3gpp.org). The network elements are functionally grouped into the Core Network (CN) 101, the UMTS Terrestrial Radio Access Network (UTRAN) 102 and the User Equipment (UE) 103. The UTRAN 102 is responsible for handling all radio-related functionality, while the CN 101 is responsible for routing calls and data connections to external networks. The interconnections of these network elements are defined by open interfaces (Iu, Uu). It should be noted that UMTS system is modular and it is therefore possible to have several network elements of the same type.
FIG. 2 illustrates the current architecture of UTRAN. A number of Radio Network Controllers (RNCs) 201, 202 are connected to the CN 101. Each RNC 201, 202 controls one or several base stations (Node Bs) 203, 204, 205, 206, which in turn communicate with the user equipments. An RNC controlling several base stations is called Controlling RNC (C-RNC) for these base stations. A set of controlled base stations accompanied by their C-RNC is referred to as Radio Network Subsystem (RNS) 207, 208. For each connection between User Equipment and the UTRAN, one RNS is the Serving RNS (S-RNS). It maintains the so-called lu connection with the Core Network (CN) 101. When required, the Drift RNS 302 (D-RNS) 302 supports the Serving RNS (S-RNS) 301 by providing radio resources as shown in FIG. 3. Respective RNCs are called Serving RNC (S-RNC) and Drift RNC (D-RNC). It is also possible and often the case that C-RNC and D-RNC are identical and therefore abbreviations S-RNC or RNC are used.
Evolved UMTS UTRAN Architecture
In the following a proposal for an Evolved UMTS UTRAN architecture will be described (see also 3GPP TR 25.897: “Feasibility Study on the Evolution of UTRAN Architecture”, V.0.2.0, available at http://www.3gpp.org). In this architecture each of the new network elements may be defined in terms of its control and user plane functions. An overview of the network architecture is given in the FIG. 9.
The RNG (Radio Network Gateway) is used for interworking with the conventional RAN, and to act as a mobility anchor point meaning that once an RNG has been selected for the connection, it is retained for the duration of the call. This includes functions both in control plane and user plane. Further, the RNG provides connectivity to the core network of the mobile communication system.
Control Plane Functions
Part of RNG functions is to act as a signaling gateway between the evolved RAN and the CN, and the evolved RAN and Rel99/4/5 UTRAN. It has the following main functions:                Iu signaling gateway, i.e. anchor point for the RANAP connection,                    RANAP (Radio Access Network Application Part) connection termination, including:                            Setup and release of the signaling connections                Discrimination of connectionless messages                Processing of RANAP connectionless messages,                                    Relay of idle and connected mode paging message to the relevant NodeB+(s),                        The RNG takes the CN role in inter NodeB+ relocations,        User plane control and        Iur signaling gateway between NodeB+ and Rel99/4/5 RNCUser Plane Functions        
The RNG is the user plane access point from the CN or conventional RAN to the evolved RAN. It has the following user plane functions:                User plane traffic switching during relocation,        Relaying GTP (GPRS tunneling protocol on the Iu interface) packets between NodeB+ and SGSN (Serving GPRS Support Node, an element of the CN) and        Iur interworking for user plane        
The NodeB+ element terminates all the RAN radio protocols (L1, L2 and L3). NodeB+ functions are studied separately for control plane and user plane.
Control Plane Functions
This category includes all the functions related to the control of the connected mode terminals within the evolved RAN. Main functions are:                Control of the UE,        RANAP connection termination,                    Processing of RANAP connection oriented protocol messages                        Control/termination of the RRC connection and        Control of the initialization of the relevant user plane connections        
The UE context is removed from the (serving) NodeB+ when the RRC connection is terminated, or when the functionality is relocated to another NodeB+ (serving NodeB+ relocation). Control plane functions include also all the functions for the control and configuration of the resources of the cells of the NodeB+, and the allocation of the dedicated resources upon request from the control plane part of the serving NodeB+.
User Plane Functions
User plane functions include the following:                Protocol functions of PDCP (Packet Data Convergence Protocol), RLC and MAC and        Macro Diversity CombiningEnhanced Uplink Dedicated Channel (E-DCH)        
Uplink enhancements for Dedicated Transport Channels (DTCH) are currently studied by the 3GPP Technical Specification Group RAN (see 3GPP TR 25.896: “Feasibility Study for Enhanced Uplink for UTRA FDD (Release 6)”, available at http://www.3gpp.org). Since the use of IP-based services become more important, there is an increasing demand to improve the coverage and throughput of the RAN as well as to reduce the delay of the uplink dedicated transport channels. Streaming, interactive and background services could benefit from this enhanced uplink.
One enhancement is the usage of adaptive modulation and coding schemes (AMC) in connection with Node B controlled scheduling, thus enhancements of the Uu interface. In the existing R99/R4/R5 system the uplink maximum data rate control resides in the RNC. By relocating the scheduler in the Node B the latency introduced due to signaling on the interface between RNC and Node B may be reduced and thus the scheduler may be able to respond faster to temporal changes in the uplink load. This may reduce the overall latency in communications of the user equipment with the RAN. Therefore Node B controlled scheduling is capable of better controlling the uplink interference and smoothing the noise rise variance by allocating higher data rates quickly when the uplink load decreases and respectively by restricting the uplink data rates when the uplink load increases. The coverage and cell throughput may be improved by a better control of the uplink interference.
Another technique, which may be considered to reduce the delay on the uplink, is introducing a shorter TTI (Transmission Time Interval) length for the E-DCH compared to other transport channels. A transmission time interval length of 2 ms is currently investigated for use on the E-DCH, while a transmission time interval of 10 ms is commonly used on the other channels. Hybrid ARQ, which was one of the key technologies in HSDPA, is also considered for the enhanced uplink dedicated channel. The Hybrid ARQ protocol between a Node B and a user equipment allows for rapid retransmissions of erroneously received data units, and may thus reduce the number of RLC (Radio Link Control) retransmissions and the associated delays. This may improve the quality of service experienced by the end user.
To support enhancements described above, a new MAC sub-layer is introduced which will be called MAC-e in the following (see 3GPP TSG RAN WG1, meeting #31, Tdoc R01-030284, “Scheduled and Autonomous Mode Operation for the Enhanced Uplink”). The entities of this new sub-layer, which will be described in more detail in the following sections, may be located in user equipment and Node B. On user equipment side, the MAC-e performs the new task of multiplexing upper layer data (e.g. MAC-d) data into the new enhanced transport channels and operating HARQ protocol transmitting entities.
Further, the MAC-e sub-layer may be terminated in the S-RNC during handover at the UTRAN side. Thus, the reordering buffer for the reordering functionality provided may also reside in the S-RNC.
E-DCH MAC Architecture at the User Equipment
FIG. 4 shows the exemplary overall E-DCH MAC architecture on user equipment side. A new MAC functional entity, the MAC-e 403, is added to the MAC architecture of Rel/99/4/5. The MAC-e 405 entity is depicted in more detail in FIG. 5.
There are M different data flows (MAC-d) carrying data packets to be transmitted from user equipment to Node B. These data flows can have different QoS (Quality of Service), e.g. delay and error requirements, and may require different configurations of HARQ instances. Therefore the data packets can be stored in different Priority Queues. The set of HARQ transmitting and receiving entities, located in user equipment and Node B respectively will be referred to as HARQ process. The scheduler will consider QoS parameters in allocating HARQ processes to different priority queues. MAC-e entity receives scheduling information from Node B (network side) via Layer 1 signaling.
E-DCH MAC Architecture at the UTRAN
In soft handover operation the MAC-e entities in the E-DCH MAC Architecture at the UTRAN side may be distributed across Node B (MAC-eb) and S-RNC (MAC-es). The scheduler in Node B chooses the active users and performs rate control by determining and signaling a commanded rate, suggested rate or TFC (Transport Format Combination) threshold that limits the active user (UE) to a subset of the TCFS (Transport Format Combination Set) allowed for transmission.
Every MAC-e entity corresponds to a user (UE). In FIG. 6 the Node B MAC-e architecture is depicted in more detail. It can be noted that each HARQ Receiver entity is assigned certain amount or area of the soft buffer memory for combining the bits of the packets from outstanding retransmissions. Once a packet is received successfully, it is forwarded to the reordering buffer providing the in-sequence delivery to upper layer. According to the depicted implementation, the reordering buffer resides in S-RNC during soft handover (see 3GPP TSG RAN WG 1, meeting #31: “HARQ Structure”, Tdoc R1-030247, available of http://www.3gpp.org). In FIG. 7 the S-RNC MAC-e architecture which comprises the reordering buffer of the corresponding user (UE) is shown. The number of reordering buffers is equal to the number of data flows in the corresponding MAC-e entity on user equipment side. Data and control information is sent from all Node Bs within Active Set to S-RNC during soft handover.
It should be noted that the required soft buffer size depends on the used HARQ scheme, e.g. an HARQ scheme using incremental redundancy (IR) requires more soft buffer than one with chase combining (CC).
E-DCH Signaling
E-DCH associated control signaling required for the operation of a particular scheme consists of uplink and downlink signaling. The signaling depends on uplink enhancements being considered.
In order to enable Node B controlled scheduling (e.g. Node B controlled time and rate scheduling), user equipment has to send some request message on the uplink for transmitting data to the Node B. The request message may contain status information of a user equipment e.g. buffer status, power status, channel quality estimate. The request message is in the following referred to as Scheduling Information (SI). Based on this information a Node B can estimate the noise rise and schedule the UE. With a grant message sent in the downlink from the Node B to the UE, the Node B assigns the UE the TFCS with maximum data rate and the time interval, the UE is allowed to send. The grant message is in the following referred to as Scheduling Assignment (SA).
In the uplink user equipment has to signal Node B with a rate indicator message information that is necessary to decode the transmitted packets correctly, e.g. transport block size (TBS), modulation and coding scheme (MCS) level, etc. Furthermore, in case HARQ is used, the user equipment has to signal HARQ related control information (e.g. Hybrid ARQ process number, HARQ sequence number referred to as New Data Indicator (NDI) for UMTS Rel. 5, Redundancy version (RV), Rate matching parameters etc.)
After reception and decoding of transmitted packets on enhanced uplink dedicated channel (E-DCH) the Node B has to inform the user equipment if transmission was successful by respectively sending ACK/NAK in the downlink.
Mobility Management within Rel99/4/5 UTRAN
Before explaining some procedures connected to mobility management, some terms frequently used in the following are defined first.
A radio link may be defined as a logical association between single UE and a single UTRAN access point. Its physical realization comprises radio bearer transmissions.
A handover may be understood as a transfer of a UE connection from one radio bearer to another (hard handover) with a temporary break in connection or inclusion/exclusion of a radio bearer to/from UE connection so that UE is constantly connected UTRAN (soft handover). Soft handover is specific for networks employing Code Division Multiple Access (CDMA) technology. Handover execution may controlled by S-RNC in the mobile radio network when taking the present UTRAN architecture as an example.
The active set associated to a UE comprises a set of radio links simultaneously involved in a specific communication service between UE and radio network. An active set update procedure may be employed to modify the active set of the communication between UE and UTRAN. The procedure may comprise three functions: radio link addition, radio link removal and combined radio link addition and removal. It should be noted that based on active set the set of NodeBs the UE is currently communicating with is identified.
The maximum number of simultaneous radio links is set to eight. New radio links are added to the active set once the pilot signal strengths of respective base stations exceed certain threshold relative to the pilot signal of the strongest member within active set.
A radio link is removed from the active set once the pilot signal strength of the respective base station exceeds certain threshold relative to the strongest member of the active set. Threshold for radio link addition is typically chosen to be higher than that for the radio link deletion. Hence, addition and removal events form a hysteresis with respect to pilot signal strengths.
Pilot signal measurements may be reported to the network (e.g to S-RNC) from UE by means of RRC signaling. Before sending measurement results, some filtering is usually performed to average out the fast fading. Typical filtering duration may be about 200 ms contributing to handover delay. Based on measurement results, the network (e.g. S-RNC) may decide to trigger the execution of one of the functions of active set update procedure (addition/removal of a Node B to/from current Active Set).
E-DCH—Node B Controlled Scheduling
Node B controlled scheduling is one of the technical features for E-DCH which is foreseen to enable more efficient use of the uplink power resource in order to provide a higher cell throughput in the uplink and to increase the coverage. The term “Node B controlled scheduling” denotes the possibility for the Node B to control, within the limits set by the RNC, the set of TFCs from which the UE may choose a suitable TFC. The set of TFCs from which the UE may choose autonomously a TFC is in the following referred to as “Node B controlled TFC subset”.
The “Node B controlled TFC subset” is a subset of the TFCS configured by RNC as seen in FIG. 8. The UE selects a suitable TFC from the “Node B controlled TFC subset” employing the Rel5 TFC selection algorithm. Any TFC in the “Node B controlled TFC subset” might be selected by the UE, provided there is sufficient power margin, sufficient data available and TFC is not in the blocked state. Two fundamental approaches to scheduling UE transmission for the E-DCH exist. The scheduling schemes can all be viewed as management of the TFC selection in the UE and mainly differs in how the Node B can influence this process and the associated signaling requirements.
Node B Controlled Rate Scheduling
The principle of this scheduling approach is to allow Node B to control and restrict the transport format combination selection of the user equipment by fast TFCS restriction control. A Node B may expand/reduce the “Node B controlled subset”, which user equipment can choose autonomously on suitable transport format combination from, by Layer-1 signaling. In Node B controlled rate scheduling all uplink transmissions may occur in parallel but at a rate low enough such that the noise rise threshold at the Node B is not exceeded. Hence, transmissions from different user equipments may overlap in time. With Rate scheduling a Node B can only restrict the uplink TFCS but does not have any control of the time when UEs are transmitting data on the E-DCH. Due to Node B being unaware of the number of UEs transmitting at the same time no precise control of the uplink noise rise in the cell may be possible (see 3GPP TR 25.896: “Feasibility study for Enhanced Uplink for UTRA FDD (Release 6)”, version 1.0.0, available at http://www.3gpp.org).
Two new Layer-1 messages are introduced in order to enable the transport format combination control by Layer-1 signaling between the Node B and the user equipment. A Rate Request (RR) may be sent in the uplink by the user equipment to the Node B. With the RR the user equipment can request the Node B to expand/reduce the “Node controlled TFC Subset” by one step. Further, a Rate Grant (RG) may be sent in the downlink by the Node B to the user equipment. Using the RG, the Node B may change the “Node B controlled TFC Subset”, e.g. by sending up/down commands. The new “Node B controlled TFC Subset” is valid until the next time it is updated.
Node B Controlled Rate and Time Scheduling
The basic principle of Node B controlled time and rate scheduling is to allow (theoretically only) a subset of the user equipments to transmit at a given time, such that the desired total noise rise at the Node B is not exceeded. Instead of sending up/down commands to expand/reduce the “Node B controlled TFC Subset” by one step, a Node B may update the transport format combination subset to any allowed value through explicit signaling, e.g. by sending a TFCS indicator (which could be a pointer).
Furthermore, a Node B may set the start time and the validity period a user equipment is allowed to transmit. Updates of the “Node B controlled TFC Subsets” for different user equipments may be coordinated by the scheduler in order to avoid transmissions from multiple user equipments overlapping in time to the extent possible. In the uplink of CDMA systems, simultaneous transmissions always interfere with each other. Therefore by controlling the number of user equipments, transmitting simultaneously data on the E-DCH, Node B may have more precise control of the uplink interference level in the cell. The Node B scheduler may decide which user equipments are allowed to transmit and the corresponding TFCS indicator on a per transmission time interval (TTI) basis based on, for example, buffer status of the user equipment, power status of the user equipment and available interference Rise over Thermal (RoT) margin at the Node B.
Two new Layer-1 messages are introduced in order to support Node B controlled time and rate scheduling. A Scheduling Information Update (SI) may be sent in the uplink by the user equipment to the Node B. If user equipment finds a need for sending scheduling request to Node B (for example new data occurs in user equipment buffer), a user equipment may transmit required scheduling information. With this scheduling information the user equipment provides Node B information on its status, for example its buffer occupancy and available transmit power.
A Scheduling assignment (SA) may be transmitted in the downlink from a Node B to a user equipment. Upon receiving the scheduling request the Node B may schedule a user equipment based on the scheduling information (SI) and parameters like available RoT margin at the Node B. In the Scheduling Assignment (SA) the Node B may signal the TFCS indicator and subsequent transmission start time and validity period to be used by the user equipment.
Node B controlled time and rate scheduling provides a more precise RoT control compared to the rate-only controlled scheduling as already mentioned before. However this more precise control of the interference at this Node B is obtained at the cost of more signaling overhead and scheduling delay (scheduling request and scheduling assignment messages) compared to rate control scheduling.
In FIG. 10 a general scheduling procedure with Node B controlled time and rate scheduling is shown. When a user equipment wants to be scheduled for transmission of data on E-DCH it first sends a scheduling request to Node B. Tprop denotes here the propagation time on the air interface. The contents of this scheduling request are information (scheduling information) for example buffer status and power status of the user equipment. Upon receiving that scheduling request, the Node B may process the obtained information and determine the scheduling assignment. The scheduling will require the processing time Tschedule.
The scheduling assignment, which comprises the TFCS indicator and the corresponding transmission start time and validity period, may be then transmitted in the downlink to the user equipment. After receiving the scheduling assignment the user equipment will start transmission on E-DCH in the assigned transmission time interval.
The use of either rate scheduling or time and rate scheduling may be restricted by the available power as the E-DCH will have to co-exist with a mix of other transmissions by the user equipments in the uplink. The co-existence of the different scheduling modes may provide flexibility in serving different traffic types. For example, traffic with small amount of data and/or higher priority such as TCP ACK/NACK may be sent using only a rate control mode with autonomous transmissions compared to using time and rate-control scheduling. The former would involve lower latency and lower signaling overhead.
Transport Channels and TFC Selection
In third generation mobile communication systems data generated at higher layers is carried over the air with transport channels, which are mapped to different physical channels in the physical layer. Transport channels are the services, which are offered by the physical layer to Medium Access Control (MAC) layer for information transfer. The transport channels are primarily divided into two types:                Common transport channels, where there is a need for explicit identification of the receiving UE, if the data on the transport channel is intended for a specific UE or a sub-set of all UEs (no UE identification is needed for broadcast transport channels)        Dedicated transport channels, where the receiving UE is implicitly given by the physical channel, that carries the transport channel        
One example for a dedicated transport channel is the E-DCH. The data is transmitted within the transport channels during periodic intervals, commonly referred to as transmission time interval (TTI). A transport block is the basic data unit exchanged over transport channels, i.e. between the physical layer and MAC layer. Transport blocks arrive to or are delivered by the physical layer once every TTI. The transport format (TF) describes how data is transmitted during a TTI on a transport channel.
The transport format consists of two parts. The semi-static part indicating the Transmission Time Interval (TTI) (e.g. 10 ms, 20 ms, 40 ms, 80 ms), the Type of FEC (Forward Error Correction) coding (e.g. convolutional, turbo, none), the Channel Coding-rate (e.g. ½, ⅓) and the CRC size. The second part, the dynamic part indicates the Number of transport blocks per TTI, and Number of bits per transport blocks.
The attributes of the dynamic part may vary for every TTI, whereas the attributes of the semi-static part are changed by RRC transport channel reconfiguration procedure. For each transport channel a set of transport formats are defined, the so-called Transport Format Set (TFS). The TFS is assigned to MAC layer from RRC at transport channel set up. An uplink or downlink connection typically consists of more than one transport channel. The combination of transport formats of all transport channels is known as the Transport Format Combination (TFC). At the start of each TTI, an appropriate TFC for all the transport channels is selected. Dependent on the number of transport channels, the TFC comprises a number of TFs, which define the transport format to be used for transmitting data of the respective transport channel within a TTI.
The MAC layer selects the transport format for each transport channel on the basis of a set of transport format combinations (or TFCS for transport format combination set) assigned by RRC radio resource control unit and also selects the quantity of data of each logical channel to be transmitted on the associated transport channel during the corresponding TTI. This procedure is referred to as “TFC (Transport Format Combination) selection”. For details on the UMTS TFC selection procedure see 3GPP TS 25.321, “Medium Access Control (MAC) protocol specification; (Release 6)”, version 6.1.0, available at http://www.3gpp.org.
TFC selection at the UE may be carried out at the start of each reference TTI, which denotes the smallest TTI of the involved transport channels. If for example TFC selection is performed among three transport channels with a TTI length of transport channel #1 equals 10 ms and a TTI length of equal to 40 ms for transport channels #2 an #3, TFC selection is performed every 10 ms.
Soft Handover (SHO) for E-DCH
It is currently considered, that soft handover is supported for data transmissions on the E-DCH, as for dedicated channels in Rel99/4/5. There are several options on how the UE uplink transmission is controlled in soft handover. There could be just one scheduling entity (scheduling Node B), which controls the uplink transmissions of the UE (maximum data rate/power ratio) or there could be multiple scheduling entities, for example all the node Bs in the active set.
E-DCH transmissions of UEs in soft handover may have an impact on the RoT variation of the multiple cells in the active set. If there is for example one Node B identified as a sole scheduling entity, scheduling of a UE in SHO without consideration of non-scheduling cells in the active set could lead to an unexpected variation of the RoT in those cells. This interference, which is also referred to as other cell interference, may have an impact on the average cell throughput in those cells.
Equation 1 shows that for a designated noise rise target in the system an increase of the i-factor—defined as the ratio of other cell interference to own cell interference—will reduce the average cell throughput. The equation can be easily derived from basic formulas presented by Holma et al., “WCDMA for UMTS”, Wiley & Sons, <year>, <chapter/pages>, 2002, Chapter 8, Page 174:
                                                                        C                _                            =                                                                    η                    UL                                    ·                  W                                                                      (                                          1                      +                      i                                        )                                    ·                                      (                                                                  E                        b                                            /                                              N                        0                                                              )                                                                                                            with                                                              i              =                                                other                  ⁢                                                                          ⁢                  cell                  ⁢                                                                          ⁢                                      interf                    .                                                                    own                  ⁢                                                                          ⁢                  cell                  ⁢                                                                          ⁢                                      interf                    .                                                                                                          Equation        ⁢                                  ⁢        1            
One way to reduce the i-factor and therefore to increase the average cell throughput is to avoid high data rate transmissions for UEs in soft handover. Only small data rates are allocated to those UEs from the scheduling Node B. But this strategy would on the other hand reduce the coverage, which is in contradiction to one of the goals of the enhanced uplink. Therefore some other method, which provides an efficient uplink noise rise management for UEs in SHO, is required.
In case of multiple scheduling Node Bs there might be some further problems. FIG. 11 shows an exemplary soft handover scenario, where both Node Bs in the active set control uplink transmissions of the UE, by restricting the maximum allowed data rate for E-DCH transmissions. Every Node B reserves resources for this UE according to the signaled maximum allowed transmission data rate.
In the example Node B2 allocates resources to the UE for a maximum data rate of 256 kps, whereas Node B1 assigns only resources up 64 kps to the UE.
The UE would for example adopt the lowest rate restriction, 64 kbps, in order to avoid causing unexpected uplink interference to Node B1. However since Node B2 is not aware of the signaled maximum allowed data rate by Node B1, it has allocated resources to the UE that will not be used. This unused resource could have been allocated to other UEs under the control of Node B2.